The invention relates to a linear RF detector comprising a detector part having a detector diode biased with bias current, and a linearizer part having an operation amplifier to the inverting input of which is connected both the output of the detector part and the feedback path coming from the output of the operation amplifier.
In a typical embodiment of the detector, the level of a received radio signal is detected, but the detector is also used for detecting the level of a transmitted radio signal. This application relates to the latter kind of detector.
Requirements set for an RF detector include good linearity, high speed and zero output voltage when the input voltage is zero. Accuracy of detection should not suffer if the detector part and the linearizer part are placed on different circuit boards.
A known detector is shown in FIG. 1. It meets all the other requirements except that the output offset voltage is not zero but depends on the differences in temperature of the detector part and the linearizer part. The detector part comprises a detector diode D1, a capacitor C and a resistor R.sub.0, and the linearizer part comprises an operation amplifier OA1 having a diode D2 and a resistor R in its feedback path. The signal whose level is to be detected is represented by generator V.sub.i cos .omega.t, where V.sub.i is the amplitude to be detected. V.sub.k is a direct-current voltage used for biasing diode D1. Starting from the universal diode current equation I.sub.D =I.sub.S (T)e.sup.Vd/Vt, where V.sub.T is a voltage proportional to absolute temperature and I.sub.S (T) is a saturation current of the diode, it is possible to show that when diode D1 has a temperature T1, whereby the saturation current is I.sub.S1 (T1), and diode D2 has a temperature T2, whereby the saturation current is I.sub.S2 (T2), the output voltage V.sub.O of the detector is represented by formula (1): ##EQU1## The first term of the formula is the desired part depending on the RF input voltage. The second term is a constant offset voltage, and the third term is a varying offset voltage depending on the temperatures and device matching of the diodes. The fourth term, where V.sub.D2 is a voltage over diode D2, generates both a temperature-dependent offset voltage and an error in that part of the output voltage which depends on amplitude V.sub.i. The part of output voltage V.sub.0 that is dependent on voltage V.sub.i is given by a non-linear function f, where I.sub.0 and I.sub.1 are Bessel functions, and is obtained from formula (2) ##EQU2## The prior art detector circuit illustrated by FIG. 1 has several advantages. First, according to formula (2) the diode bias current I.sub.b has theoretically, i.e. if the diode functions as an ideal diode, no effect on the part f(V.sub.i, V.sub.T1) of the output voltage V.sub.O that is dependent on the input voltage. The bias current will have to be set at a large enough value to enable fast charge/discharge of the circuit capacitances. On the other hand, although according to the simplified theory the performance of the circuit is not critical with respect to the bias current of the diode, there are, however, secondary influences, and so the bias current will have to be within a certain range. Second, resistor R.sub.0 does not affect the output voltage in the transfer of the input voltage. Third, the detector is fast, if R.sub.0 and C are sufficiently small. The fourth advantage is that the linear dynamic range of the detector is about 50 dB. The lowest input level to a 50 .OMEGA. impedance is -20 dBm (sensitivity dV.sub.0 /dV.sub.i has dropped to half of its nominal value) and the highest input level is +30 dBm, depending on the break down voltage of diode D1 and supply voltage of the operation amplifier.
The main disadvantage of the above-described known detector circuit concerns the output offset voltages, the values of which are predictable only if diodes D1 and D2 are matched devices and have the same temperature, so they should preferably be on the same silicon chip. However, it is often advantageous to place the detector and the linearizing amplifier on different circuit boards. It should also be noted that if diodes D1 and D2 are in the same package, RF energy will easily leak through diode D2 into the linearizing amplifier disturbing its operation. This is difficult to prevent, since D2 is in the feedback path of a fast amplifier, in the vicinity of which no filtering is allowed.
A way of eliminating the offset voltage in a circuit according to FIG. 1 is shown in FIG. 2. In the latter figure, offset compensation is used outside the feedback path of the linearizing amplifier. Diodes D1a and D2a are matched devices and have the same temperature, and they are preferably in the same package. Biasing voltage V.sub.k is obtained by the use of a diode D2 that is similar to diodes D1a and D1b. This makes the bias current independent of the forward voltage and the temperature of the diodes. The offset voltage of matched diodes D1a and D1b at the output of the first operation amplifier OA1 is 2*V.sub.k. The non-inverting amplification of the latter operation amplifier OA2 is 2 and the inverting amplification is 1, so the final offset at output V is 2*V.sub.k -1*2V.sub.k =O.
The circuit of FIG. 2 has exactly the same drawbacks as the circuit of FIG. 1: the detector and the linearizing amplifier must be placed on the same circuit board, which raises EMC (Electro-Magnetic Compatibility) problems when the RF input level is high (more than 10 dBm). The RF energy leaks through diode D1b into linearizer OA1 and is rectified in the p-n junctions of diode D1b and/or of the operation amplifiers. It is almost impossible to arrange an effective lowpass filter in the vicinity of D1b, because the diode is in the feedback path of a fast amplifier. An extra operation amplifier reduces the speed of the detector and raises the price, especially since the detector must be fast.